Light-curing substances based on acrylate/methacrylate undergo volume shrinkage during radical polymerization due to the reduced molecular distance resulting from the polymerization and the associated increase in density. This shrinkage may be significantly reduced by adding inorganic fillers such as dental glass or pyrogenic silicic acids, since a reduced monomer proportion per unit volume is obtained, and the fillers do not shrink during the polymerization.
Volume shrinkage has great clinical importance for dental applications, since tensile forces are transmitted to the cavity wall as the result of material shrinkage. When a maximum force is exceeded, in extreme cases this shrinkage force may lead to detachment from the cavity wall. Bacteria may infiltrate the peripheral gap thus created, causing secondary dental caries.
An observation of the progression of the shrinkage force over time results in the following typical findings:
Directly after the polymerization, the volume shrinkage results in an initial value for the shrinkage force which then increases to a maximum value within about 24 hours due to post-polymerization. Absorption of water (in the laboratory, from storage in water, or in the mouth, from saliva) after several days to weeks causes a slight volume expansion of the composite, and the stress forces may relax once again and return to a lower level.
As a result, the decisive influencing variable is the maximum shrinkage stress value after approximately 24 hours, since this parameter represents the maximum force load of the combined composite/adhesive/tooth system.
There have been many attempts to provide low-shrinkage dental materials: DE 199 05 093 A1 proposes the use of bicyclic monomers which cure via ring-opening metathesis polymerization (ROMP). According to DE 198 51 038 A1, the shrinkage is controlled by adding acryloylmorpholine, cumarone resin, vinyl stearate, polyvinyl acetate, or alcohol surfactants before polymerization. According to U.S. Pat. No. 5,750,590, cationically polymerizable “oxetanes” (trimethylene oxides) have low shrinkage and therefore are also suitable for reduced-shrinkage dental materials. U.S. Pat. No. 6,855,197 B2 describes reduced-shrinkage filler materials based on epoxy resin, containing nanoscale inorganic oxides as fillers. According to U.S. Pat. No. 6,709,271 B2, use of a filler mixture composed of spherical fillers having a particle size of 200-500 nm and submicron fillers having a particle size of 20-80 nm results in shrinkage of up to 1.8% following polymerization.
The present patent application relates primarily to the shrinkage force and the reduction of same. In addition to the material properties described above by way of example, the shrinkage force also influences processing parameters:
Luminous Power
A light curing device having a pulse mode operation for eliminating shrinkage force problems is proposed in DE 199 13 890 A1.
Polymerization kinetics: For identical composite materials, lower shrinkage forces may be achieved by an initially slower polymerization at lower luminous power, followed by an increase in the luminous power to the maximum value (soft-start polymerization). As a result of the lower luminous power at the beginning, the composite material remains flowable for a longer period, and therefore is better able to compensate for and reduce stress (J. Esthet. Restor. Dent. (2003) 15, 93-104). In US 20050065227 A1 it is presumed that in the use of multifunctional photoinitiators the early stages of shrinkage occur as long as the material is still elastic, ultimately resulting in lower shrinkage stress.
Geometry of the restorative: Shrinkage forces may be minimized by use of an incremental technique in building the restorative (U.S. Pat. No. 6,783,810 B2). However, the more layers that must be individually cured, the more time required by the dentist to provide treatment.